Spectroscopically Identified Intermediate Age Stars at 0.5 - 3 pc Distance from Sgr A*

Abstract

<Context.> Nuclear star clusters (NSCs) at the dynamical center of galaxies appear to have a complex star formation history. This suggests repeated star formation even in the influence of the strong tidal field from supermassive black holes. <Aim.> In our previous study, we have detected 31 so far unknown early-type star candidates throughout the Galactic NSC (at 0.5 - 3 pc from Sgr A*; Nishiyama and Schoedel 2013). The aim of this study is a confirmation of the spectral type for the candidates. <Method.> We have carried out NIR spectroscopic observations of the candidates using Subaru/IRCS/AO188/LGS. K-band spectra for 20 out of the 31 candidates were obtained. By determining an equivalent width, EW(CO), of the 12CO absorption feature at 2.294 um, we have derived an effective temperature and a bolometric magnitude for each candidate, and then constructed an HR diagram. <Results.> No young (~ Myr), massive stars are included in the 20 candidates we observed; however, 13 candidates are most likely intermediate-age giants (50 - 500 Myr). Two other sources have ages of ~1 Gyr, and the remaining five sources are old (> 1 Gyr), late-type giants. <Conclusions.> Although none of the early-type star candidates from our previous narrow-band imaging observations can be confirmed as a young star, we find that the photometric technique is sensitive to distinguish old, late-type giants from young and intermediate-age populations. The intermediate-age stars could be so far unknown members of a population formed in a starburst ~100 Myr ago. Finding no young (~ a few Myr) stars at R = 0.5 - 3 pc favors the in-situ formation scenario for the presence of the young stars at R < 0.5 pc. Furthermore, the different spatial distributions of the young and the intermediate-age stars imply that the Galactic NSC is an aggregate of stars born in different places and under different physical conditions.

9 pages, 8 figures, 2 tables; accepted for publication in A&A

References in corpus (8)

• The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation
• The ACS Virgo Cluster Survey. VIII. The Nuclei of Early-Type Galaxies
• The Coincidence of Nuclear Star Clusters and Active Galactic Nuclei
• Nuclear star clusters in 228 spiral galaxies in the HST/WFPC2 archive: catalogue and comparison to other stellar systems
• Evidence for a Long-Standing Top-Heavy IMF in the Central Parsec of the Galaxy
• A new stellar library in the region of the CO index at 2.3 $\mu$m. New index definition and empirical fitting functions
• Massive Star Formation in the Molecular Ring Orbiting the Black Hole at the Galactic Center
• Mapping the Outer Edge of the Young Stellar Cluster in the Galactic Center

Cited by in corpus (11)

• The distribution of old stars around the Milky Way's central black hole I: Star counts
• KMOS view of the Galactic Centre - II. Metallicity distribution of late-type stars
• Spectroscopic Detection of a Cusp of Late-type Stars around the Central Black Hole in the Milky Way
• Near-Infrared Variability Study of the Central 2.3 arcmin x 2.3 arcmin of the Galactic Centre I. Catalog of Variable Sources
• Smooth kinematic and metallicity gradients reveal that the Milky Way's nuclear star cluster and disc might be part of the same structure
• A spectroscopic map of the Galactic centre -- Observations and resolved stars
• A Wide Metallicity Range for Gyr-old Stars in the Nuclear Star Cluster
• The Near-Infrared Spectrum of the Nuclear Star Cluster: Looking Below the Tip of the Iceberg and Comparisons with Extragalactic Nuclei
• Age and metallicity of the Milky Way's nuclear star cluster studied at 3 pc from Sagittarius A*
• Supergiant GCIRS 22 in the Milky Way Nuclear Star Cluster: Revised alpha abundances
• Photometric properties of nuclear star clusters and their host galaxies in the Fornax cluster